I like BOOTS approach and would like to add something that has not been said, have a helper hold the correct attachment on the end of a shop vacuum hose (with the filter in the vacuum cleaner clean of course) to capture MOST of the metal/metal dust. Mark H.

Run it.... I ran Venolia 2618's with the partial head of a broken valve imbedded in the piston for 1/2 season in one of my 351-c's. Big race was coming up, I had a new piston on order but it was Sunday AM and we raced Tuesday. After a couple good finishes the new piston became the spare and we never took the time to replace the damaged one.

Last place in the B-main is better than anyplace in the grandstands...

Sometimes we are not that lucky to have a choice to replace a part and are committed to reusing something that is less than ideal,,that said knocking the sharp edges off the damage on the crown in the o/p's case so as not to cause an ignition problem would do the job ,i know its not pretty but its not critical ,if that damage translated into some kind of precursor of failure somewhere down the track i would be surprised, as its pretty much a skid mark and when you think about it any machining on a piston crown is closely controlled skid marks [machining] if the damage was caused by an impact that could compromise the crowns integrity that would be a matter for more investigation. Keeping any swarfe contamination out of the engine would be more important than anything else.[all of this stated previously]

My paragraph opener above leads to this part,, some years back an engine that is used for land speed events was damaged when fired up after a lengthy period of storage [methanol engine ,valve corroded in guide ,valve opens ,doesn't close ,engine fired immediately ,much crunch, junk thrown into the manifold and into the other cylinder ] anyway we ordered another set of pistons urgently as we only had just over a week till the bike would be on the lake, and was assured they were sitting on the pilots lap on their way here , after all the promises they let us down pretty badly . I now have a guy that has driven 1400km to my workshop with no top on his motor and no prospect of getting parts to do the job,with two days to complete his journey [he has another 500km to get to the track] he had sent me the heads weeks earlier so all the crunch damage was repaired and finished on them ,i left the bores till i had pistons in my hands before i would machine them ,as stated previously ,no pistons ,so, we looked closely at the damaged ones ,there was no puncture damage through the crowns but the ring lands were distorted from the crunch damage.
We machined the chamber profile over the shrapnel damage and were lucky enough that the pistons were originally supplied with a metric 1.5mm ring pack and i had a set of imperial rings that were for the same bore so we machined the bottom lands flat so the rings had a better seat , a hone with keyway stones took the high spots out of the bores ,we still had a heap of dents though.

The motor went back together with some pretty ordinary parts but that was that ,no choice.The rider was instructed to keep his fingers on the clutch lever and any hint of failure ,clutch it and just let the motor go,the motor did not let go and went on to set a new record for 1350APS-PF of 180mph at Lake Gairdner South Australia

Sometimes its not careful planning it's just pure luck, but you wouldn't tempt it twice.

Cheers.

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